I buy king's court and chapel more than I buy silver. In fact, I can rarely see a point in not buying chapel. I'm not as enthusiastic about chapel as I used to be and there are certainly situations where I won't buy it but those situations are rarer than you would think. I've learned to see chapel as an enabler for other strategies rather than a strategy in of itself and there's usually something out there that benefits from chapel.
King's Court to me can often seem a trap when you're at $8+. The way I see it, there has to be a card that is worth playing three times that is likely to collide with King's Court, and that has to be worth delaying a Province for.
For example, on a Mountebank board, I'd often spend that $8 on Province instead. Sure, King's Court-Mountebank is powerful, but more likely the two cards will miss each other in an increasingly diluted deck, and there's a good chance only one to three Provinces will be bought the whole game, so I can't risk passing up the Province.
As another example, there might be a province board with lots of strong non terminal drawing (Stables, Laboratory, etc.), but with no strong terminals and with no +buy. On that board, King's Court is kind of a waste, as at best its going to enable $8+ later. Gold will give as much chance of hitting $8, and a Province whenever possible is going to be best. In that circumstance I might not pick up a single King's Court.
As for Chapel, I probably leave it behind more than most, as I'm a big user of Big Money + 2 terminal draw strategies, which tend to be slowed by Chapels. Also I tend to like Mint as a fast trasher in some decks, as unlike Chapel it stays useful in the mid to late game. In other words, if the dominant strategy looks to be money based rather than engine based, I'll pass on Chapel.
Not to say both Chapel and King's Court aren't strong cards, but I think to say that they're ALWAYS worth taking is untrue.
Familiar. Looks so cheap for such awesomeness.
Now this one is far from a must have. The problem here is Potion. Potion can mess up so many hands that might otherwise have been strong.
As a rule of thumb, if there's another curser on table, i'll ignore Familiar. I'd rather climb for $5 and get Mountebank or Witch, or pass up the potion and get a Sea Hag.
Re: suggestions in general, I think you'd have to ask yourself another question: If this card had been Embargoed, would I still pick it up?
Or for something like Familiar, if the potion gets embargoed, would you still buy the potion to go for Familiar?
If the answer is no, then you wouldn't be buying this card every time its available
